Firstly, try 'reset docking layout' from File -> DesignSpark options -> Appearance.
If that has no effect, click 'reset all user settings' in File -> DesignSpark options -> Advanced.
If your file tab has gone missing or it looks like:
Delete the 'SpaceClaim' folder located at the path %LocalAppData%\SpaceClaim and launch DS Mechanical again. The icons and layouts should now be fixed.
After getting back the missing tabs/options, switch the appearance by heading into File (or, top left DSM logo) -> DesignSpark options. Change the ribbon style to Windows 8 (Office 2013).